SM resistors having flat resistive elements are produced in two main configurations—lead-less, and with leads. In the first configuration, the resistors are lead-less rectangular chips with termination pads for electrical connection at the two opposite ends of a resistive layer on top of a ceramic substrate.
The first configuration of SM resistor may have different termination configurations, including “Wrap Around Metallization” or “Flip Chip” configured termination. In chips with “Wrap Around Metallization”, the pads extend from the termination pads over the short sides of the rectangular chip and to part of the chip's bottom. Soldering to a PCB is performed according to one of these two configurations of the terminations. For chips having pads on top only, a “Flip Chip” configuration, with pads down. For chips with “Wrap Around Metallization” the chip's metallized bottom is placed on the pads of the PCB.
The second configuration of a surface mount resistor is molded in a resin package with two flat leads attached to the termination pads, extending on two sides of the package, and bent to seat, for soldering, on the pads of a PCB.
The first configuration has the advantage of smaller dimensions and lower manufacturing costs, but has a limitation of failures of the solder joint to the PCB's pads when subjected to high mechanical and/or thermal stresses, the latter due to a mismatch of Coefficients of Thermal Expansion (CTE) between the chip's ceramic substrate and the PCB's material. The probability of such failure increases with the size of the chip.
Such stresses occur when the ambient temperature changes and the chip does not expand or contract at same rate as the PCB, or when a load applied to the resistor causes a rise of its temperature while the PCB remains cooler, or when the PCB is flexed causing a change of the distance between its pads. Thus, despite advances in the art, problems remain.
What is needed is a solution to failures of chip resistors caused by mechanical and thermal stresses after the chips are soldered to a Printed Circuit Board (PCB).
Therefore, it is a primary object, feature, or advantage of the present invention is to provide a low-cost method, suitable for mass production, of transforming lead-less chip resistors into devices with flexible leads without significant increase of their size.